Selective recruitment of m RNA s and mi RNA s to polyribosomes in response to rhizobia infection in M edicago truncatula

Summary Translation of m RNA s is a key regulatory step that contributes to the coordination and modulation of eukaryotic gene expression during development or adaptation to the environment. m RNA stability or translatability can be regulated by the action of small regulatory RNA s (s RNA s), which control diverse biological processes. Under low nitrogen conditions, leguminous plants associate with soil bacteria and develop a new organ specialized in nitrogen fixation: the nodule. To gain insight into the translational regulation of m RNA s during nodule formation, the association of m RNA s and s RNA s to polysomes was characterized in roots of the model legume M edicago truncatula during the symbiotic interaction with S inorhizobium meliloti . Quantitative comparison of steady‐state and polysomal m RNA s for 15 genes involved in nodulation identified a group of transcripts with slight or no change in total cellular abundance that were significantly upregulated at the level of association with polysomes in response to rhizobia. This group included m RNA s encoding receptors like kinases required either for nodule organogenesis, bacterial infection or both, and transcripts encoding GRAS and NF ‐ Y transcription factors ( TF s). Quantitative analysis of s RNA s in total and polysomal RNA samples revealed that mature micro RNA s (mi RNA s) were associated with the translational machinery, notably, mi R 169 and mi R 172, which target the NF ‐ YA / HAP 2 and AP 2 TF s, respectively. Upon inoculation, levels of mi R 169 pronouncedly decreased in polysomal complexes, concomitant with the increased accumulation of the NF ‐ YA / HAP 2 protein. These results indicate that both m RNA s and mi RNA s are subject to differential recruitment to polysomes, and expose the importance of selective m RNA translation during root nodule symbiosis.